Machines such as wheel loaders, excavators, off-highway vehicles, locomotives, power generators, and the like, are typically equipped with an engine system including a combustion engine to produce power. The combustion engine may be associated with an air intake system configured to draw air into a combustion chamber of the engine for combustion, and an exhaust system for discharging exhaust gases produced after the combustion to the atmosphere. The combustion engine typically includes a number of engine cylinders, each associated with a combustion chamber having a number of valves, such as, intake valves and exhaust valves. Typically, intake valves are connected with the air intake system, and may open and close to allow and block air flow from the air intake system to the cylinders. Exhaust valves are typically connected with an exhaust manifold, or exhaust common rail, in the exhaust system to discharge exhaust gases. Exhaust valves may open and close to allow and block exhaust gas flow from the combustion chamber to the exhaust manifold. During an engine cycle, the intake valves and the exhaust valves may be opened and closed at determined times.
Under some circumstances, a boost in engine power may be demanded by the machine to which the engine is providing power. For example, more power may be demanded when a mobile machine undergoes a sudden acceleration, or when the mobile machine needs to overcome a large load, etc. In order to obtain additional power from an existing engine system without modifying the engine capacity, energy recovering assemblies may be utilized to recover energy which could otherwise be wasted from exhaust gases. Such energy recovering assemblies may include turbo compounding devices, turbochargers, and exhaust waste heat recovery devices. Both turbo compounding devices and turbochargers receive exhaust gases to drive an associated turbine, thereby converting the kinetic energy of the exhaust gases into the mechanical energy of the turbine, which may be utilized by other devices such as a compressor to compress air. Exhaust waste heat recovery devices can recover thermal energy from the heat of exhaust gases, and convert the thermal energy into other forms of energy, such as mechanical or electrical energy. Use of exhaust waste heat recovery devices may improve overall efficiency of the engine.
A turbo compounding device typically includes a turbine and a power coupling device. When exhaust gases from the engine reach the turbine, the exhaust gas flow can cause the turbine to rotate. Thus, the kinetic energy of the exhaust gases can be converted into the mechanical rotating energy of the turbine. Through a power coupling device, which may share a common rotating shaft with the turbine, the energy of the rotating turbine can be coupled with a drive output device of the engine, adding additional power to the total engine power output. A typical power coupling device may be mechanical, and may include a number of gears that couple the rotating turbine with a crankshaft of the engine. However, a power coupling device may also be electrical, converting the rotating mechanical energy of the turbine into electrical energy.
A typical turbocharger includes a turbine and a compressor drivingly connected with each other through a common rotating shaft. The exhaust gases from the engine drive the turbine to rotate, which in turn causes the compressor to rotate through the common rotating shaft. The rotating compressor then draws air from the atmosphere, compresses the air, and drives the compressed air into the air intake system of the engine. With the air being compressed, more air and fuel can be drawn into the engine for combustion during an engine cycle. As a result, more power can be produced by the combustion engine.
When a turbo compounding device or a turbocharger is used to recover energy from exhaust gases, the turbine of such devices may generate a back pressure, forming a resistance against the exhaust gas flow. This back pressure may affect the performance of the engine. Under some circumstances, high back pressure created by the turbine can adversely affect engine performance. For example, due to the resistance of turbine back pressure, an increased amount of exhaust residual may be left in the combustion chamber (i.e., the cylinder) during an exhaust stroke of an engine cycle. As a result, pumping work of the piston may be increased, and energy may be lost in the increased pumping work. Furthermore, increased exhaust residual in the combustion chamber also occupies space, resulting in a reduced amount of air taken into the chamber during an air intake stroke. With less air for combustion, less power is produced in a normal engine cycle, which may result in reduced engine combustion efficiency.
Under other circumstances, high back pressure can be beneficial during an engine cycle, and therefore may be desirable. For example, high back pressure created by the turbine may help increase the positive work done by the piston during an expansion stroke. During engine blow down, when the exhaust valves open during the late portion of the expansion stroke, the high back pressure may act on the piston and increase engine power output.
An internal combustion engine with a system for controllably opening and closing exhaust and intake valves is described in U.S. Pat. No. 6,460,337 (the '337 patent) issued to Olofsson on Oct. 8, 2002. The system disclosed in the '337 patent includes an engine with a plurality of engine cylinders each including exhaust and intake valves, and a turbocharger to utilize energy of exhaust gases to compress air. The exhaust valves are divided into a first and a second group of exhaust valves connected by respective first and second exhaust manifolds. The first exhaust manifold directs exhaust gases from the first group of exhaust valves to a turbine of the turbocharger, and the second exhaust manifold directs exhaust gases from the second group of exhaust valves to a catalyst through an exhaust pipe.
While the '337 patent may provide an improved internal combustion engine, the improvement is mainly achieved through effective air charging by utilizing the turbocharger when engine speed increases. To do so, the times for opening/closing the intake and exhaust valves are changed such that the temperature increase resulting from compression in the cylinders is reduced. With such a reduction in the temperature increase, engine combustion efficiency may be improved. However, when considering exhaust energy recovery, the system of the '337 patent may have drawbacks. The system of the '337 patent only includes a single turbocharger to recover energy from the exhaust gases for supercharging, and does not disclose any other exhaust energy recovery devices, such as turbo compounding systems. Therefore, the efficiency of exhaust energy recovery by the system may be limited. Furthermore, although the system includes divided exhaust-gas discharge through the first and second groups of exhaust valves, the portion of exhaust gases from the second group of exhaust valves is simply discharged through the exhaust pipe without passing through any energy recovery devices. This portion, which could contain a significant amount of the total energy produced during an engine cycle, is thus wasted in the system of the '337 patent.
The system and method of the present disclosure are directed toward improvements in the existing technology.